Today, hydraulic systems are widely used by manufacturing, construction, power generation, mining and transportation industries. Over the years, systems for the harnessing and distribution of power have become increasingly sophisticated, their applications more numerous and their operating conditions more demanding. Hydraulic systems are particularly advantageous in that they allow for actuation of large surfaces under heavy loads with minimal input, due to the fact that hydraulic fluid resists compression (i.e. incompressible) and therefore facilitates the direct transfer of applied work to the actuated surfaces. Hydraulic systems also offer an advantage of being more powerful than an electrical system of the same size, particularly in heavy load applications.
Hydraulic systems involving reciprocating piston pumps and motors can provide efficient power transfer mechanisms in alternative energy conversion systems, however hydraulic fluid movement between the hydraulic pump and motor can be problematic during system start-up and other non-steady state operating conditions. Priming of a reciprocating piston hydraulic pump in a open loop can be very difficult, as the ability for the pump piston to draw its own hydraulic fluid into the respective bore inlet is limited due the force required to overcome inertia in circulation of the hydraulic fluid between the pump and reservoir. This problem in draw ability of the pump is analogous to the age-old problem of pushing a rope, as the pump must overcome inertia of the hydraulic fluid in the system in order to begin operation.
The priming (and other non-steady state conditions) problem is exacerbated if the hydraulic reciprocating piston pump is located at a head height above the reciprocating piston hydraulic motor and reservoir tank in the open loop system, as the piston must draw the hydraulic fluid under further influence of gravity. This problem of inertia is further exaggerated by the pump size (i.e. bore/stroke), inlet hydraulic fluid volume (i.e. bore inlet diameter) and/or separation between the pump and motor increase(s) in magnitude, or if the pump piston (s) decouples from the actuator driving the pump.
The current solution employed for the aforementioned problems is the provision of an open loop hydraulic system, in which a reservoir is positioned at higher altitude than the pump and the pump pistons are fixed to the actuator (do not decouple) or in some applications, a supplemental pump or other priming device is used to supply hydraulic fluid to the main, more efficient pump. However, the open loop system is undesirable, as the supplemental pump increases the complexity and can decrease the efficiency of the hydraulic system. Also, in applications where space and accessibility to the hydraulic pump(s) are limited or difficult, the use of an open loop storage reservoir system is unworkable and undesirable.
One exemplary application of pump-motor hydraulic systems is for a hydraulic wind turbine, where blade rotation is converted to hydraulic flow using a hydraulic pump, pressure of hydraulic flow is generated by an opposing load or resistance, and the hydraulic pressure is converted to electrical energy using a hydraulic motor coupled to an electrical generator.